1. Field of the Invention
The invention disclosed herein relates generally to the circuit configuration and packaging configuration of power MOSFETs. More particularly, this invention relates to a novel and improved circuit diagram for preventing shoot through problem by using a shunt FET and different configurations for integrating the shunt FET.
2. Description of the Prior Art
Conventional power MOSFET devices still face the shoot through problems that result in excessive dissipation and efficiency loss. Referring to FIG. 1 for a circuit diagram of a conventional buck converter 10 that includes a high side MOSFET 15 and a low side MOSFET 20 serially connected between an input terminal 25 having an input voltage represented by Vin and a ground terminal 30. The drain of the low side MOSFET 20 is connected to the source of the high side MOSFET 15 at a mid point 35 connecting to the load 40 through inductance L and capacitance C. When the buck converter 10 operates at high speed, a shoot through condition becomes a problem when both the high side and low side MOSFET are turned on at the same time causing a shoot through current to flow between the input terminal 25 and the ground terminal 30. The shoot through condition results in excessive dissipation and efficiency loss. In order to avoid the shoot through problem, a controlling circuit 45 is implemented to control the gate signals to generate a dead time between the gate signals for the high side and low side MOSFET. FIG. 2 shows such a dead time between the time when the high side MOSFET 15 is turned off and the time when the low side MOSFET 20 is turned on such that the high side and low side MOSFETs are prevented from turning on simultaneously.
However, the shoot through problem cannot be completely avoided due to the fact that a large drain current is generated at the low side MOSFET 20 when the high side MOSFET 15 is turned on as shown in FIG. 3 due to a large rate of change of the voltage, i.e., dV/dt, at the mid-connection point 35. FIG. 4 shows an equivalent circuit of the buck converter wherein the drain current generated flows through the gate-drain capacitor Cgd and then to the ground through the internal gate-source capacitor Cgs or through an equivalent circuit segment comprises gate resistor Rg. inductor Lg, and external gate drive resistance Rext. Under such circumstances, if the impedance from the gate to the ground is not below a certain value then the drain current, i.e., Cdg*dV/dt, will generate a voltage drop across the gate of the low side MOSFET that would be large enough to turn on the low side MOSFET 20 thus inducing shoot-through. In modern circuit designs, a designer typically controls the problem by using a large gate-source capacitance Cgs or a low Crss/Ciss ratio. Alternately, the problem may also be prevented by providing a low gate resistance and using a high current gate drive with low Rext. However, if the gate drive circuitry, i.e., the control circuit 45, is remote from the MOSFET, the inductance Lg may become quite large. This causes the current path connected with Rg, Rext, and Lg to have great impedance thus leaving only the Cgs path to sink the transient current. The only way to suppress the shoot through current is by increasing the capacitance Cgs to reduce the impedance. However, this solution will lead to excessive gate charge losses in the low side MOSFET 20. For the above reasons, a person of ordinary skill of the art is faced with limitations and difficulties in designing a converter to effectively prevent the shoot through problem.
Therefore, a need still exists in the art to provide an improved device configuration and manufacturing methods to make MOSFET devices with a very low impedance path for the Cdg*dV/dt current. The low impedance suppresses the gate-source voltage spike, and thus prevents shoot-through problems and resolve the above discussed difficulties as now encountered in the prior art.